Build and install extra packages

[bennyrowland]

What's the problem this feature will solve?

This is basically the same request as #1162. In the comtypes package we have sufficiently complicated test build requirements that we would like to have a second package in the repo to build the test components, the resulting wheel could then be installed in the test environment and used for testing. This could be done using pre commands, but I would prefer to have a proper isolated build environment to make the package, as it will have a number of build dependencies.

@gaborbernat suggested in #1162 that this could be done with a plugin (and offered to provide some assistance for that). I would be very happy to develop such a plugin, but don't yet have enough understanding of the tox details to see how to design one. I would therefore welcome some guidance on how perhaps to get started on this, and maybe some thoughts on a suitably tox-style interface. @proofit404 - do you still have any interest in this issue?